1 Why is the metric system used instead of the English system and vice versa in various parts of the world?

Answer 1

The conversion factors are a lot simpler. Everything's related by powers of ten, so you don't have to stuff your brain with nonsense like "12 inches in a foot, 3 feet in a yard, 5.5 yards in a rod, 4 rods in a furlong, 8 furlongs in a mile."

The only significant part of the world where the English system is still officially used is the US (granted, it's a pretty significant part of the world), and the reason is ... well, nominally it's because it would cost a lot to retool factories for metric units ... this wasn't as big a factor in most other countries, because they had no industrial facilities to speak of in the first place (the Third World), or because they were rebuilding them anyway what with them having been destroyed in a war and all (most of Europe), or because they switched before the industrial revolution was in full swing (other parts of Europe), or because the economic benefits of doing so outweighed the disadvantages (still other parts of Europe). The US economy is internally so big that for many US companies international trade is nice, but it's not strictly speaking necessary. Also, the US is enough of an economic powerhouse that even in international trade it can essentially say to other countries "You're getting your wheat measured in bushels instead of octolitters or whatever the heck you call them, and if you don't like it you can just starve."